Process for producing polyimide/metallic foil composite film

ABSTRACT

A polyimide/metallic foil composite film having a curvature radius of at least 25 cm and which is substantially free from curling is described, comprising coating a solution of a polyimide precursor in an organic polar solvent on a 1 to 500 μm thick metallic foil, the polyimide precursor being prepared by reacting a diamine component comprising p-phenylenediamine and an aromatic tetracarboxylic acid component comprising 3,3&#39;,4,4&#39;-biphenyltetracarboxylic dianhydride or its derivative; heat-drying the above-prepared coating in the state that the metallic foil is fixed; and then heating it at a high temperature to form a 5 to 200 μm thick polymide film. This composite film does not substantially curl in either the lengthwise and widthwise directions and is very suitable for use in the production of an electrical circuit board.

This is a division of application Ser. No. 716,313 filed Mar. 27, 1985,now U.S. Pat. No. 4,623,563.

FIELD OF THE INVENTION

The present invention relates to a process for producing apolyimide/metallic foil composite film.

BACKGROUND OF THE INVENTION

A composite film comprising a metallic foil with a polyimide layerlaminated thereon is useful for an electric circuit board. Suchcomposite films have been produced by the following procedures: (A) amethod comprising bonding a polyimide film to a metallic foil through anadhesive; (B) a method comprising heat-bonding a polyimide film to ametallic foil; and (C) a method comprising applying a solution of apolyimide precursor in an organic polar solvent onto a metallic foil,drying the coating and imidating the polyimide precursor to form apolyimide layer.

The method (C) has several advantages as compared to the methods (A) and(B), such as that the process can be simplified because it is notnecessary to form a film in advance as in the methods (A) and (B), athin composite film can be produced, and the process is free fromtroubles due to the use of an adhesive as in the method (A). In themethod (C), however, the coating shrinks when cooled after heating fordrying or imidating, and curling occurs in composite films thus producedbecause it is difficult for the metallic foil to conform to theshrinkage of the coating. Thus, the composite films of method (C) have adisadvantage in that they cannot be used in the preparation of electriccircuits, or that they are not convenient to use for such preparation.

SUMMARY OF THE INVENTION

Extensive investigations have been made to overcome the above-describedproblems.

Accordingly, an object of the present invention is to provide a processfor producing a polyimide/metallic foil composite film which issubstantially free from curling without requiring the formation of thepolyimide films in advance.

The process for producing a polyimide/metallic foil composite filmaccording to the present invention comprises:

coating a solution of a polyimide precursor in an organic polar solventonto a metallic foil having a thickness of from 1 to 500 μm, thepolyimide precursor being prepared by reacting a diamine componentcomprising p-phenylenediamine with an aromatic tetracarboxylic acidcomponent comprising 3,3',4,4'-biphenyltetracarboxylic dianhydride or aderivative thereof,

drying the above-prepared coating by heating in the state that themetallic foil is fixed, and

heating the coating at a high temperature to form a polyimide filmhaving a thickness of from 5 to 200 μm.

Thereby there is obtained a polyimide/metallic foil composite filmhaving a curvature radius of 25 cm or more and which is substantiallyfree from curling in the lengthwise and widthwise directions.

BRIEF DESCRIPTION OF THE DRAWING

The drawing illustrates a cross-section view of a polyimide/metallicfoil composite film, which is provided to explain the curvature radius.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention can produce a polyimide/metallicfoil composite film having a curvature radius of 25 cm or more which issubstantially free of curling in both the lengthwise and widthwisedirections. The reasons that such a composite film can be produced arethat the polyimide coating prepared from the above-specified diamine andaromatic tetracarboxylic acid components has a coefficient of linearthermal expansion which is nearly equal to that of a metallic foilcomprising, for example, copper or aluminum, and that, in the formationof the polyimide coating on the metallic foil, a heat-drying process anda high-temperature heating process including imidation are carried outin the state that the metallic foil is fixed; in particular, thehigh-temperature heating process releases the stress formed in thepolyimide layer.

The composite film produced by the process of the present invention canbe used as substrates for the production of electric circuit boardswithout any problems. The composite film has the advantage that thehandling properties in processing the circuits are excellent. Anotheradvantage is that the electric circuit boards obtained are resistant tocurling due to temperature changes, and therefore the boards haveexcellent dimensional stability.

The term "curvature radius" as used herein is described below. Thedrawing shows a cross-sectional view of a polyimide/metallic compositefilm 3 comprising a metallic foil 1 and a polyimide coating 2. Thiscomposite film 3 has a length of 10 cm and a width of 10 cm. When thecomposite film 3 curls in the widthwise (or lengthwise) direction, thecurvature radius is defined as a radius r, a distance from a centralpoint P. Assuming that a distance from a point R to the center of thecomposite film 3 when it curls is h, if the distance h is equal to orlarger than the radius r, i.e., h≧r, the curvature radius is determinedby actually measuring the radius r. The symbol M indicates a horizontalline connecting the both extremities of the composite film 3 in thecurled condition, and the length of the horizontal line M is representedby a. The symbol N indicates a perpendicular line relative to thehorizontal line M and crosses with the horizontal line M at the point R.On the other hand, if H<r, the curvature radius is determined byactually measuring the distance h and the length a, and then calculatingfrom the following equation.

    r.sup.2 =(r-h).sup.2 +(1/2a).sup.2

    r.sup.2 =r.sup.2 -2rh+h.sup.2 +1/4a.sup.2

    2rh=h.sup.2 +1/4a.sup.2

    r=1/2h+1/8·a.sup.2 /h

The composite film of the present invention has the relationship of h<rand r=25 cm or more, preferably at least 50 cm, and more preferably ∞.That is, the present invention provides a composite film substantiallyfree from curling.

The diamine component which can be used to prepare the polyimideprecursor or polyimide comprises p-phenylenediamine. The preferreddiamine component comprises 80 mol% or more of p-phenylenediamine and 20mol% or less of other diamines. If the proportion of p-phenylenediamineis too small, the difference in coefficient of linear thermal expansionbetween the polyimide layer and the metallic foil comprising, forexample, copper or aluminum increases, which is undesirable.

Other diamines which can be used include 4,4'-diaminodiphenyl ether,4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone,3,3'-diaminodiphenylsulfone, m-phenylenediamine,4,4'-diaminodiphenylpropane, 1,5-diaminonaphthalene,2,6-diaminonaphthalene, 4,4'-diaminodiphenyl sulfide,4,4'-di(m-aminophenoxy)diphenylsulfone,3,3'-di(m-aminophenoxy)diphenylsulfone, and4,4'-di(m-aminophenoxy)diphenylpropane. These compounds can be usedalone or as mixtures thereof. Several mol% of diaminosiloxane may beused.

The aromatic tetracarboxylic acid component which can be used to preparethe polyimide precursor comprises 3,3',4,4'-biphenyltetracarboxylicdianhydride or its derivative such as acid halide, diester, andmonoester. The preferred aromatic tetracarboxylic acid componentcomprises 70 mol% or more of the above dianhydride or derivative and 30mol% or less of other aromatic tetracarboxylic dianhydrides or theirderivatives such as acid halides, diesters, and monoesters. If theproportion of 3,3',4,4'-biphenyltetracarboxylic dianhydride or itsderivative is too small, various problems undesirably occur such thatthe difference in coefficient of linear thermal expansion between thepolyimide layer and the metallic foil increases, or the strength of thecoating deteriorates remarkably.

Other aromatic tetracarboxylic dianhydrides and their derivatives whichcan be used include pyromellitic dianhydride,3,3',4,4'-benzophenonetetracarboxylic dianhydride, and2,3,6,7-naphthalenetetracarboxylic dianhydride, and their derivatives.These compounds can be used alone or as mixtures thereof. Of thesecompounds pyromellitic dianhydride and its derivatives, and3,3',4,4'-benzophenonetetracarboxylic dianhydride and its derivativesare preferably used. The reason for this is that even if they arereacted alone with the above-specified diamine component, it isdifficult to produce a polyimide coating having an excellent coatingstrength, but good results can be obtained in reducing the coefficientof linear thermal expansion, and this is particularly suitable for theobject of the present invention of preventing curling.

In preparing the polyimide precursor such as polyamide acid, preferablyapproximately equimolar amounts of the diamine and aromatictetracarboxylic acid components are reacted in an organic polar solventat 0° to 90° C. for 1 to 24 hours.

Organic polar solvents which can be used for this purpose includeN-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide,dimethylsulfoxide, dimethylphosphoamide, m-cresol, p-cresol, andp-chlorophenol. In addition, solvents such as xylene, toluene, hexane,and naphtha may be used in combination with the above solvents.

The polyimide precursor thus-prepared preferably has a logarithmicviscosity (measured at 30° C. in N-methyl-2-pyrrolidone at aconcentration of 0.5 g/100 ml) of from 0.4 to 7.0 and preferably from1.5 to 3.0. If the logarithmic viscosity is too small, the polyimidecoating has a low mechanical strength. On the other hand, if it is toolarge, the coating workability on the metallic foil is poor.

According to the process for producing the composite film of the presentinvention, a solution of the polyimide precursor in the organic polarsolvent is heated to a temperature of 80° C. or less to decrease itsviscosity, and then is flow coated onto the metallic foil, which has athickness of from 1 to 500 μm, preferably from 10 to 100 μm, and morepreferably from 20 to 50 μm, by a suitable means such as use of anapplicator which can adjust the coating thickness. If the thickness ofthe metallic foil is less than 1 μm, the curling is difficult toprevent, and the composite film is unsuitable for practical use. On theother hand, if the thickness thereof is more than 500 μm, the compositefilm has a poor flexibility and is unsuitable for use as, for example,an electric circuit board.

Examples of the metallic foil are a copper foil and an aluminum foil. Inthe case of the copper foil, an electrolytic copper foil, a rolledcopper foil, or an electrolytic or rolled copper foil which has beenfurther surface treated with a silane coupling agent or analuminum-based coupling agent is preferably used in view of the factthat such a foil has a large adhesion to the polyimide layer. Inaddition, foils of metals such as silver, iron, a nickel/chromium alloy,and stainless steel can also be used. The length of the metallic foil inthe above-described polyimide precursor solution is not particularlylimited. The width of the metallic foil is generally from about 20 to200 cm for most practical purposes, but is not limited to theabove-specified range. Moreover, the composite film produced using themetallic foil having a width falling within the above-specified rangemay be cut to a predetermined width at the final step and used.

The same organic polar solvent as used in each of the polymerizationreaction to prepare the polyimide precursor can be used as the organicpolar solvent used for the preparation of the polyimide precursorsolution. The concentration of the polyimide precursor in the solutionis from about 10 to 20% by weight. If the concentration is too low, thesurface of the polyimide coating becomes rough. On the other hand, ifthe concentration is too high, the viscosity of the resulting solutionincreases and the solution becomes difficult to coat. From thestandpoint of each coating, the viscosity of the solution at the time ofcoating is preferably 1,000 poises or less. Further, in order toincrease the adhesion between the metallic foil and the polyimidecoating, a silane coupling agent may be coated on the metallic foil ormay be added to and mixed with the above coating solution.

After the solution is coated on the metallic foil, the coating is driedby heating at a temperature of from 100° to 230° C. for from 30 minutesto 2 hours to remove the solvent. Subsequently the temperature isincreased and, finally, the coating is preferably heated at atemperature of from 230° to 600° C. for from 1 minute to 6 hours, andmore preferably near the glass transition temperature of the polyimideformed, i.e., from 250° to 350° C. for from 10 minutes to 6 hours tothereby complete the imidation reaction and simultaneously remove thesolvent and release the stress formed in the polyimide coating duringthe imidation reaction.

If the heat treatment to achieve the imidation and release the stress iscarried out at temperatures below 230° C., release of the stress isinsufficient and the composite film tends to curl. On the other hand, ifthe heat treatment is carried out at temperatures more than 600° C., thepolyimide is decomposed. In order to prevent the decomposition of thepolyimide, it is preferred that the heating time at temperatures morethan 350° C. be controlled to less than 10 minutes.

The above heat-drying process and high-temperature heat treatment arecarried out in the state that the metallic foil with the polyimideprecursor solution coated thereon is fixed. The fixing methods of themetallic foil include various procedures which can substantially fix themetallic foil in both the widthwise and lengthwise directions dependingon the length and size of the metallic foil, for example, a method inwhich the metallic foil is fixed to a glass plate in a flat-plate formby using a polyimide tape, and a method in which the metallic foil isfixed by winding both lengthwise extremities of the metallic foil arounda cylinder.

After the above heat-drying process and high-temperature heat treatment,the metallic foil with the polyimide coating is cooled to roomtemperature. The fixation may be removed any time after thehigh-temperature heat treatment. It is preferred that the fixation beremoved after the metallic foil with the polyimide coating is cooled toroom temperature.

The polyimide coating wherein the stress is released satisfactorily isformed on the metallic foil. The thickness of the polyimide coating isfrom 5 to 200 μm, preferably from 10 to 100 μm, and more preferably from10 to 50 μm. If the thickness of the polyimide layer is less than 5 μm,the composite film has poor film characteristics. On the other hand, ifthe thickness is more than 200 μm, it is difficult to prevent thecurling, and the composite film has poor flexibility and is unsuitablefor use as an electric circuit board.

The polyimide coating generally has an average coefficient of linearthermal expansion at the temperature range of from 50° to 250° C. in therange of from 1.2×10⁻⁵ to 2.9×10⁻⁵ /°C. It is possible to decrease theaverage coefficient of linear thermal expansion. On the other hand, theaverage coefficient of linear thermal expansion of the metallic foilhaving a thickness of from 1 to 500 μm as determined for the sametemperature range as above is in the range of from 1.5×10⁻⁵ to 1.7×10⁻⁷/°C. for a copper foil and in the range of from 2.4×10⁻⁵ to 2.6×10⁻⁵/°C. for an aluminum foil.

The present invention thus preferably has the characteristic that thedifference in average coefficient of linear thermal expansion betweenthe polyimide coating and the metallic foil within the above-describedtemperature range can be controlled within 0.3×10⁻⁵ /°C. or less byappropriately selecting the thickness of each of the polyimide coatingand the metallic foil, and the polymer composition of the polyimidecoating within the above-specified ranges.

The coefficient of linear thermal expansion is represented by:

    Δl/l

wherein l is a length of a material at a temperature T and Δl is achange in length by the 1° C. temperature change. The averagecoefficient of linear thermal expansion is defined as an average valueof coefficients of linear thermal expansion within a given temperaturerange. The coefficient of linear thermal expansion is measured asfollows:

The composite film is cut to form a test piece having a length of 25 mmand a width of 3 mm. One extremity in lengthwise direction of the testpiece is fixed as an upper extremity, and a load of 15 g/mm² is appliedto the lower extremity with a distance between chucks of 10 mm. In thiscondition, the test piece is placed in a nitrogen gas atmosphere and thetemperature is changed at a temperature-raising rate of 10° C./min.

The thus-produced polyimide/metallic foil composite film has a curvatureradius of at least 25 cm, preferably at least 50 cm, and more preferably∞ in both the widthwise and lengthwise directions, and is substantiallyfree from curling. Moreover, the composite film has excellent heatresistance, chemical resistance, durability, and flexibility, and alsohas an excellent adhesion between the polyimide coating and the metallicfoil. Therefore, the composite film can be suitably used as a printedwiring substrate, a flexible printed wiring substrate, a multi-layerwiring substrate, or an oscillation plate. Even if the composite film issubjected to a processing treatment in which the film is generallyheated to a temperature of from 50° to 270° C., the film issubstantially free from curling after cooling. Thus, the composite filmof the present invention has excellent handling properties anddimensional stability.

The present invention is described in greater detail by reference to thefollowing examples.

The curvature radius and the average coefficient of linear thermalexpansion were measured and calculated in the same manner as describedabove, using test pieces cut off from composite films produced in eachof the examples and comparative examples. The curvature radius wasmeasured in both the lengthwise and widthwise directions of a 10×10 cmtest piece and the values measured were substantially equal to eachother.

EXAMPLE 1

10.8 g (0.1 mol) of p-phenylenediamine and 210 g ofN-methyl-2-pyrrolidone (hereinafter abbreviated to "NMP") was introducedinto a 500 ml flask to dissolve in the diamine in the NMP. To theresulting solution was gradually added 29.4 g (0.1 mol) of3,3',4,4'-biphenyltetracarboxylic dianhydride while stirring. Duringthis period, the reaction system was cooled with ice water so that thetemperature did not exceed 30° C. Thereafter, the mixture was stirredfor 2 hours to prepare a 16.1% by weight NMP solution of polyamide acid.The logarithmic viscosity of the polyamide acid (measured at 30° C. inNMP at a concentration of 0.5 g/100 ml) was 2.32. The viscosity of theNMP solution was 1,820 poises (30° C.).

The NMP solution of polyamide acid was heated to decrease the viscosityto 1,000 poises or less and flow coated by an applicator which canadjust the coating thickness on a 35 μm thick copper foil having a sizeof from 30 cm×20 cm which had been fixed to a glass plate of the samesize as that of the copper foil with a polyimide film at all theextremities and heated at 150° C. for 30 minutes, at 200° C. for 60minutes, and then at 270° C. for 2 hours. The composite film was cooledto room temperature and the fixation of the copper foil was removed.

The polyimide/copper foil composite film thus-produced had a thicknessof the polyimide coating of 24 μm and a curvature radius of 82 cm, andwas substantially free from curling.

The 90° peeling strength between the polyimide layer and the copper foilin this composite film was 1.48 kg/10 mm at room temperature (e.g.,20°-30° C.), and 1.40 kg/10 mm after immersing the composite film in asoldering bath maintained at 260° C. for 30 seconds. Moreover, thecomposite film did not curl even if pattern etching was applied thereon.

The coefficient of linear thermal expansion of the polyimide layer inthe composite film was measured by a thermal mechanical analysis(hereinafter abbreviated to "TMA"). The average coefficient of linearthermal expansion at the temperature of 50° to 250° C. was 1.62×10⁻⁵/°C., and this value was nearly equal to the average coefficient oflinear thermal expansion (1.60×10⁻⁵ /°C.) of the copper foil at the sametemperature range as above.

EXAMPLES 2 TO 5

The NMP solution of polyamide acid as prepared in Example 1 was flowcoated in the same manner as in Example 1 on a 35 μm thick copper foilof the same size as in Example 1, which had been fixed on a glass plateof the same size as the copper foil in the same manner as in Example 1,heated at 150° C. for 30 minutes and at 200° C. for 60 minutes, thenheated under the conditions shown in Table 1, followed by cooling toproduce a polyimide/copper foil composite film in which the thickness ofthe polyimide coating was 24 μm. The curvature radius of the compositefilm is shown in Table 1. The curvature radius of the composite filmproduced in Example 1 is also shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                Heating Condition                                                                             Curvature                                                       Temperature   Time    Radius                                        Run No.   (°C.)  (hours) (cm)                                          ______________________________________                                        Example 2 230           12      25                                            Example 3 250           6       56                                            Example 1 270           2       82                                            Example 4 280           2       ∞                                       Example 5 300           1       ∞                                       ______________________________________                                    

COMPARATIVE EXAMPLE 1

A 19.0 wt% NMP solution of polyamide acid was prepared in the samemanner as in Example 1 except that 20.0 g (0.1 mol) of4,4'-diaminodiphenyl ether was used in place of 10.8 g (0.1 mol) ofp-phenylenediamine. The logarithmic viscosity of the polyamide acid(measured at 30° C. in NMP at a concentration of 0.5 g/100 ml) was 2.12,and the viscosity of the NMP solution was 2,040 poises (30° C.).

This NMP solution of polyamide acid was flow coated in the same manneras in Example 1 on a 35 μm thick copper foil of the same size as inExample 1, which had been fixed to a glass plate of the same size as thecopper foil in the same manner as in Example 1, heated under the sameconditions as in Example 1, and then cooled to room temperature. Thefixation of the copper foil was removed. The polyimide/copper foilcomposite film thus-produced had a thickness of the polyimide layer of29 μm and a curvature radius of 0.8 cm, and curled greatly.

The average coefficient of linear thermal expansion of the polyimidecoating in the composite film measured by the TMA at the temperaturerange of from 50° to 250° C. was 3.4×10⁻⁵ /°C., and this value waslarger than that of the copper foil at the same temperature range asabove. For this reason, it is believed that the composite film wouldcurl when cooled to room temperature even if the stress was released atthe time of forming the polyimide coating.

COMPARATIVE EXAMPLE 2

The NMP solution of polyamide acid as prepared in Comparative Example 1was flow coated in the same manner as in Example 1 on a 35 μm thickcopper foil of the same size as in Example 1, which had been fixed to aglass plate of the same size as the copper foil in the same manner as inExample 1, heated at 150° C. for 30 minutes, at 200° C. for 60 minutesand at 300° C. for 1 hours, and then cooled to room temperature. Thefixation of the copper foil was removed. The polyimide/copper foilcomposite film thus-produced had a thickness of the polyimide coating of24 μm and a curvature radius of 1.1 cm, and curled greatly.

EXAMPLES 6 TO 9

A 500 ml flask was charged with a solvent and a diamine component asshown in Table 2, and the diamine component was dissolved in thesolvent. The amount of the solvent used was such that the concentrationsof the diamine and aromatic tetracarboxylic acid components were each15% by weight.

To the solution thus prepared was gradually added an aromatictetracarboxylic acid component as shown in Table 2 while stirring.During this period, the reaction system was cooled with ice water sothat the temperature did not exceed 30° C. The mixture was then stirredfor a certain time to prepare a polyamide acid solution having alogarithmic viscosity (measured at 30° C. in NMP at a concentration of0.5 g/100 ml), as shown in Table 2.

The polyamide acid solution thus-prepared was flow coated in the samemanner as in Example 1 onto a copper foil having the same size as inExample 1 and a thickness as shown in Table 2, which had been fixed to aglass plate of the same size as the copper foil in the same manner as inExample 1, heated at 150° C. for 30 minutes, at 200° C. for 60 minutesand at 300° C. for 2 hours, and then cooled to room temperature. Thefixation of the copper foil was removed. The thickness of the polyimidelayer and the curvature radius of the composite film were as shown inTable 2. The difference between the polyimide coating and the copperfoil in the average coefficient of linear thermal expansion measured byTMA in the temperature range of from 50° to 250° C. is shown in Table 2.

COMPARATIVE EXAMPLES 3 TO 7

Polyamide acid solutions having a logarithmic viscosity (measured at 30°C. in NMP at a concentration of 0.5 g/100 ml) as shown in Table 2 wereprepared using solvents, diamine components, and aromatictetracarboxylic acid components as shown in Table 2 in the same manneras in Examples 6 to 9.

These polyamide acid solutions were used to produce polyimide/copperfoil composite films in the same manner as in Examples 6 to 9. Thesecomposite films were measured for the curvature radius, the thickness ofeach of the copper foil and polyimide coating, and the differencebetween the copper foil and the polyimide layer in the averagecoefficient of linear thermal expansion was measured in the temperaturerange of from 50° to 250° C. The results obtained are shown in Table 2.

In Table 2, the abbreviations are as follows.

p-PDA: p-phenylenediamine,

m-PDA: m-phenylenediamine,

DADE: 4,4'-diaminodiphenyl ether,

DADM: 4,4'-diaminodiphenylmethane,

S-BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride

PDA: pyrromellitic dianhydride,

BTDA: 3,3',4,4'-benzophenonetetracarboxylic dianhydride,

DMF: N,N-dimethylformamide.

                                      TABLE 2                                     __________________________________________________________________________                                                           Difference                                                                    in Average                                                  Thickness         Coefficient                            Aromatic             of    Thickness   of Linear                     Diamine  Tetracarboxylic      Copper                                                                              of    Curvature                                                                           Thermal                       Component                                                                              Acid Component                                                                              Logarithmic                                                                          Foil  Polyimide                                                                           Radius                                                                              Expansior              Run No.                                                                              (amount: mol)                                                                          (amount: mol)                                                                          Solvent                                                                            Viscosity                                                                            (μm)                                                                             (μm)                                                                             (cm)  (/°C.)          __________________________________________________________________________    Example 6                                                                            p-PDA                                                                              (0.1)                                                                             s-BPDA                                                                             (0.08)                                                                            DMF  1.9    35    14    72    0.28 ×                                                                  10.sup.-5                              PDA  (0.02)                                                   Example 7                                                                            p-PDA                                                                              (0.1)                                                                             S-BPDA                                                                             (0.08)                                                                            DMF  1.8    35    12    92    0.20 ×                                                                  10.sup.-5                              BTDA (0.02)                                                   Example 8                                                                            p-PDA                                                                              (0.08)                                                                            S-BPDA                                                                             (0.1)                                                                             NMP  2.2    35    22    88    0.21 ×                                                                  10.sup.-5                     DADE (0.02)                                                            Example 9                                                                            p-PDA                                                                              (0.08)                                                                            s-BPDA                                                                             (0.1)                                                                             NMP  2.3    70    61    91    0.14 ×                                                                  10.sup.-5                     m-PDA                                                                              (0.02)                                                            Comparative                                                                          p-PDA                                                                              (0.05)                                                                            s-BPDA                                                                             (0.1)                                                                             NMP  1.9    35    11    5      1.4 ×                                                                  10.sup.-5              Example 3                                                                            DADE (0.05)                                                            Comparative                                                                          p-PDA                                                                              (0.05)                                                                            PDA  (0.1)                                                                             DMP  2.0    35    25    3      1.8 ×                                                                  10.sup.-5              Example 4                                                                            m-PDA                                                                              (0.05)                                                            Comparative                                                                          p-PDA                                                                              (0.05)                                                                            BTDA (0.1)                                                                             DMF  1.7    35    36    1.6    2.2 ×                                                                  10.sup.-5              Example 5                                                                            DADM (0.05)                                                            Comparative                                                                          DADE (0.1)                                                                             PDA  (0.1)                                                                             NMP  2.1    70    41    0.8    2.6 ×                                                                  10.sup.-5              Example 6                                                                     Comparative                                                                          DADE (0.1)                                                                             BTDA (0.1)                                                                             DMF  1.6    70    31    0.6    2.4 ×                                                                  10.sup.-5              Example 7                                                                     __________________________________________________________________________

EXAMPLE 10

An NMP solution of polyamide acid was prepared in the same manner as inExample 1, except that 9.73 g (0.09 mol) of p-phenylenediamine and 2.0 g(0.01 mol) of 4,4'-diaminodiphenyl ether was used as the diaminecomponent. The logarithmic viscosity of the polyamide acid (measured at30° C. in NMP at a concentration of 0.5 g/100 ml) was 2.1.

The above-prepared NMP solution of polyamide acid was flow coated in thesame manner as in Example 1 on a 5 μm thick aluminum foil of the samesize as in Example 1, which had been fixed to a glass plate of the samesize as the aluminum foil in the same manner as in Example 1, heated at150° C. for 30 minutes, at 180° C. for 60 minutes and at 290° C. for 2hours, and then cooled to room temperature. The fixation of the aluminumfoil was removed. The polyimide/aluminum foil composite filmthus-produced had the thickness of the polyimide coating of 26 μm andthe curvature radius of 76 cm.

The average coefficient of linear thermal expansion of the polyimidelayer as determined by TMA at the temperature range of from 50° to 250°C. was 2.3×10⁻⁵ /°C., and this value was nearly equal to the averagecoefficient of linear thermal expansion (2.5×10⁻⁵ /°C.) of the aluminumfoil at the same temperature range as above.

It can be seen from the above Examples and Comparative Examples that theprocess of the present invention can produce polyimide/metallic foilcomposite films which are substantially free from curling.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A polyimide/metallic foil composite film having acurvature radius of at least 25 cm and which is substantially free fromcurling, comprising a metallic foil having a thickness of 1 to 500 μmand a polyimide film which is prepared from a polyimide precursor whichcomprises a diamine component comprising p-phenylenediamine and anaromatic tetracarboxylic acid component comprising3,3',4,4'-biphenyltetracarboxylic dianhydride or a derivative thereof.